Coated abrasive grains and a manufacturing method therefor

ABSTRACT

Coated abrasive grains and a manufacturing method for them are disclosed. The abrasive grains are coated using an immersion method with a coating comprising at least one substance selected from carbides, borides, and nitrides of a metal, comprising: 
     preparing an immersion bath comprising a molten salt bath containing said metal; and 
     immersing abrasive grains to be treated in said immersion bath for a suitable length of time.

This is a divisional of allowed application No. 07/260,752 filed Oct.21, 1988.

BACKGROUND OF THE INVENTION

This invention relates to coated abrasive grains and to a manufacturingmethod therefor, and particularly to coated abrasive grains obtained byan immersion method. More specifically, it relates to coated abrasivegrains and a manufacturing method therefor in which a molten salt bathis employed, and the abrasive grains are coated with a layer of a metalcompound such as at least one substance selected from metallic carbides,metallic borides, and metallic nitrides.

Abrasive grains such as diamond abrasive grains and hard BN abrasivegrains are referred to as ultrahard abrasive grains and are widely usedas starting materials for abrasive articles or as abrasives. Inparticular, abrasive grains comprising natural or synthetic diamonds arebonded together with a binder in the form of a metal, a synthetic resin,or a vitreous inorganic material to form abrasive articles, such asgrinding wheels. These abrasive articles have a wide range of usesincluding the grinding of carbide tools and hard materials such asceramics, ferrite, and glass, and the cutting of stones and concrete. Asa result of the spectacular development of ceramics in recent years,diamond wheels and hard BN wheels, which enable high-precision grindingof ceramics, have been used in large quantities.

However, presently-used abrasive grains have the following problems.

For example, in the case of resin-bonded wheels or metal-bonded wheelswhich have diamond abrasive grains as their main component, in order toincrease the bond strength between the abrasive grains and the resin ormetal which serves as a binder, the surfaces of the abrasive grains arecoated with a metal such as nickel. This metal coating is said to havethe effect of preventing the deterioration of the resin due to heatwhich is generated along the edges of the abrasive grains during use ofa grinding wheel. However, the metal coating has the problem that itcauses the clogging of the wheel.

On the other hand, vitrified-bonded wheels which also have diamondabrasive grains as their main component use a ceramic as a binder. Asthese wheels are manufactured by firing at high temperatures, thebonding strength between the abrasive grains and the binder is high dueto a solid phase reaction. However, during the high-temperature firing,the diamond abrasive grains undergo thermal corrosion, and as a resultmanufacture is difficult.

Japanese Published Unexamined Patent Application No. 55-162499 (1980)disclosed a method of coating the surface of diamond abrasive grainswith Ti(C,N,O) by chemical vapor deposition in order to preventoxidation of the abrasive grains during manufacture of wheels, improvethe adhesion to coatings, and prevent the clogging of the wheel duringgrinding. However, the adhesion of a coating formed by this method issaid to be inadequate.

Thus, as the advantages of coated abrasive grains have been recognized,various methods have been proposed for improving the properties thereof.However, as all of these methods employ CVD or other vapor phasedeposition processes, they are unsuitable for mass production, and theyrequire large and expensive equipment in order to be carried out.

The following U.S. patents are issued regarding coated diamond abrasivegrains of the prior art: U.S. Pat. Nos. 3,465,416; 3,520,667; 3,617,346;3,650,714; 3,826,630; 3,924,031; 3,929,432; 4,063,907; 4,220,455;4,417,906; and 4,606,738.

Accordingly, in order to take advantage of the above-described excellentproperties of coated abrasive grains, there is a need for a method whichcan easily and greatly improve the adhesion to coatings of coatedabrasive grains and which at the same time is inexpensive.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide ultrahardcoated abrasive grains and a method for the manufacture, thereof, theabrasive grains having superior oxidation resistance, wear resistance,and adhesion to coatings.

Another object of the present invention is to provide ultrahard coatedabrasive grains having the above properties and a method for theirmanufacture which is less expensive and more suitable for massproduction than CVD.

In Japanese Published Unexamined Patent Application No. 62-27577 (1987),one of the present invention proposed a method for forming a hardcoating on the surface of a metal using a molten chloride bath. It wasfound that this method greatly increases both the hardness of thesurface of the metal and the wear resistance.

As a result of subsequent research, it was found that this method isapplicable not only to metals but to nonmetals as well. Particularly inthe case of nonmetals, this method not only improves hardness, but theadhesion of the coating to the surface is remarkably increased. This isbecause it employs a molten salt bath, and a portion of the surface tobe treated is roughened.

Upon further investigation concerning the application of an immersionmethod for accomplishing the above objective, it was found that coatingthe surface of abrasive grains produces remarkable effects, and thepresent invention was completed.

Accordingly, the present invention provides coated abrasive grainscomprising abrasive grains whose surfaces are coated by an immersionmethod with a coating comprising at least one substance selected fromcarbides, borides, and nitrides of a metal.

The term "immersion method" here refers to a method in which abrasivegrains are immersed in a molten salt bath to form a surface coating.

The present invention also provides a manufacturing method for coatedabrasive grains in which a coating comprising at least one substanceselected from carbides, borides, and nitrides of a metal is formed onthe surfaces of abrasive grains by an immersion method, wherein animmersion bath comprising a molten salt bath containing the metal isprepared and abrasive grains to be treated are immersed in the immersionbath for a suitable length of time.

The coated grains of the present invention can be used without furthertreatment, or if necessary, they may be subjected to after-treatment byfurther coating, such as by plating, of a metallic compound, a metal, oran alloy.

In preferred embodiments of the present invention, the abrasive grainsare diamond abrasive grains or hard BN grains.

When forming a metallic carbide coating, and particularly when theabrasive grains are hard BN abrasive grains, it is preferable to usehard BN abrasive grains on which carbon was previously deposited byvapor deposition. Furthermore, when a thick metallic carbide coating isdesired, diamond abrasive grains previously coated with a carbon coatingby carbon vapor deposition or other method can be used. The carbon inthe surface layer of the abrasive grain reacts with the metal in theimmersion bath to form a carbide coating. Alternatively, if uncoatedhard BN abrasive grains are immersed, B is substituted by a metal in theimmersion bath by a substitution reaction, and the resulting coatingcomprises a nitride of the metal coming from the immersion bath.

The molten salt bath preferably comprises at least one substanceselected from chlorides of an alkali metal and chlorides of an alkaliearth metal. In a further preferred embodiment, the molten salt bath isa fluoride-containing chloride bath. The fluoride may comprise at leastone substance selected from fluorides of an alkali metal and fluoridesof an alkali earth metal.

In accordance with a different mode of the invention, the molten saltbath may be a fluoride-containing molten bromide bath, afluoride-containing molten iodide bath, or a fluoride-containing moltenfluoride bath, and the immersion bath may be prepared by adding one ormore of oxides of a metal, and halides of a metal, a metal in elementalform, and alloys thereof to the molten salt bath. In this case, thefluoride, the bromide, or the iodide is preferably a fluoride, abromide, or an iodide of an alkali metal or an alkali earth metal.

In a preferred embodiment, the immersion bath is prepared by adding anoxide and/or halide of a metal together with a metal in elemental formor an alloy thereof to the molten salt bath.

When the immersion treatment is performed with different types ofmetals, it can be preformed two or more times.

In accordance with yet another mode of the invention, a manufacturingmethod for abrasive grains coated with a boron compound is provided inwhich an immersion bath is prepared by adding at least one of an oxideof boron, a boron alloy, and a boron-containing carbide (B₄ C),preferably an oxide of boron and a boron alloy (or boron carbide) to afluoride-containing molten halide bath, and uncoated abrasive grains orabrasive grains previously coated with a substance selected from Ti, Cr,V, W, Mo, Zr, Hf, Nb, Ta, Ni, and alloys or compounds thereof areimmersed in the immersion bath for a suitable length of time. Coatedabrasive grains of the present invention may of course be used insteadof the above-described metal-coated abrasive grains.

The "coating" of abrasive grains with a metal (a metal in elementalform, a metal alloy, or a metal compound) does not necessarily refer tocompletely enveloping the abrasive grains in the metal or to plating itwith the metal. It is necessary only that the metal be applied to atleast the surface to be treated. Many different methods can be employedfor applying the metal, including electroplating, electroless plating,cladding, physical vapor deposition (sputtering, etc.), and chemicalvapor deposition (vapor plating, etc.). There is no restriction on theshape of the abrasive grains.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing the oxidation resistance of conventionaluncoated diamond abrasive grains;

FIG. 2 is a graph showing the oxidation resistance of coated diamondabrasive grains in accordance with this invention;

FIGS. 3a and 3b are photomicrographs (×45) showing the surface conditionof diamond abrasive grains coated with TiC and of peeled material,respectively, after a peeling test;

FIGS. 4a and 4b are photomicrographs (×45) showing the surface conditionof uncoated diamond abrasive grains and of peeled material,respectively, after a peeling test;

FIG. 5 is an electron microscope photograph (×1500) of the surface of apiece of the coated abrasive grains shown in FIG. 3; and

FIG. 6 is a photograph (×3000) taken with an X-ray microanalyzer showingthe distribution of elemental Ti in the same coated abrasive grains.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be explained more concretely.

A molten chloride bath, which is a representative example of a moltensalt bath which constitutes the immersion bath employed in the presentinvention, generally comprises KCl-BaCl₂ to which a fluoride such as NaFis added. Other examples of chlorides which can be employed for themolten chloride bath are NaCl, LiCl, and CaCl₂, and other examples offluorides are NaF, KF, LiF, CaF₂, and BaF₂. Preferably, the bathcomprises a chloride of an alkali metal, a chloride of an alkali earthmetal, and a fluoride of an alkali metal. The exact proportions of thecomponents will be readily apparent to those skilled in the art from theforegoing explanation. However, with a KCl-BaCl₂ -NaF bath, theproportions are generally 5-95 mole %, of KCl, 5-95 mole % of BaCl₂, and5-50 mole % of NaF. If the proportion of the fluoride is greater than 50mole %, not only does the bath temperature become too high, but alsocorrosion becomes a problem.

Alternatively, a bromide, an iodide, or a fluoride can be used in placeof the chloride in the molten salt bath. As in the case in which achloride is employed, the bromide, the iodide, or the fluoride ispreferably selected from bromides, iodides, and fluorides of an alkalimetal or an alkali earth metal.

There is no particular restriction on the type of metal which is addedto the molten bath. However, as one of the purposes of the presentinvention is to increase the adhesion of a coating to the abrasivegrains being treated, hard metals which have in the past been thoughthard to form into coatings are preferred, some examples of which are Si,Cr, V, B, W, Mo, Ti, Zr, Hf, Nb, Ta, and other members or groups IVa,Va, and VIa of the periodic table.

It is advantageous that a portion of the metal which is added to themolten salt bath be in the form of a compound (typically an oxide),since metallic compounds are easy to obtain and convenient to handle. Itis also advantageous that another portion of the metal be in the form ofa metal powder. The metal powder may be in elemental form or in the formof an alloy, such as ferroalloy.

When sodium fluoride is used as the fluoride, the NaF and the oxidesreact in the molten salt bath containing potassium and/or sodium, andpotassium and/or sodium metal fluorides are formed partially. Thesesubstances are formed by reactions such as TiO₂ →NaK₂ TiF₆, Cr₂ O₃→NaCrF₃, V₂ O₅ →Na₃ VF₆, B₂ O₃ →KBF₄, and WO₃ →K₃ WF₆. The resultingsubstances then react on the surfaces of the abrasive grains andrespectively become Ti-C (TiC), Cr-C (Cr₇ C₃, Cr₃ C₂, Cr₂₃ C₆), V-C (V₂C, V₄ C₃, V₈ C₇, VC₀.88), B-C, W-C, and the like. At this time, themetal in elemental or alloyed form is thought to act as a reducing agentfor the metal which was added to the bath in the form of a metallicoxide.

There is no restriction on the amount of the metal to be added inelemental or alloyed form, but is is adequate if an oxide of such ametal constitutes 2 weight % of the immersion bath. If the amount of themetal is too small, a coating of sufficient thickness can not beobtained, and under usual conditions the lower limit is about 1 weight%. On the other hand, if the amount of metal is too large, the thicknessof the resulting carbide coating becomes nonuniform. Under usualconditions, the upper limit is about 7 weight %. Preferably, the amountof metal is 5-7 weight %.

The metal may be added to the molten salt bath in the form of a halide.

When such a metallic halide is used, the molten salt bath may contain upto 40 weight % of the metallic halide. If the amount of metallic halideexceeds this level, the corrosion of equipment becomes a problem, and ahealth problem is created by the generation of large amounts of halogengas.

When a metal which is added to the molten salt bath is in the form of anoxide, a reducing agent for the oxide is also added to the bath.Examples of reducing agents are substances having a greater affinity foroxygen than the metal, e.g., chromium or group Va elements, such as Mn,Al, Ca, Si, Ti, and Zr, as well as alloys thereof such as Fe-Mn, Fe-Al,Fe-Ti, Fe-Zr, Fe-Si, Ca-Si, and Ca-Si-Mn. The reducing agent mayconstitute 2-20 weight % of the treatment bath. If less than 2 weight %is added, an adequate reducing effect is not obtained, and if more than20 weight % is added, the formation of carbides is obstructed.Preferably, the proportion of the reducing agent is 5-15 weight %.

The immersion time and temperature for the immersion method of thepresent invention vary in accordance with the type of metal compoundwhich is to be formed, but in general it is sufficient to performimmersion at 700°-1000° C. for from one to several hours.

The ultrahard abrasive grains which are employed in the presentinvention are preferably diamond abrasive grains, hard BN abrasivegrains, or a mixture thereof. There is no particular restriction on thetype of diamond abrasive grains which are employed, and abrasive grainsof either natural or synthetic diamonds may be used. There is noparticular limitation on grain size, but when the diamond or hard BNabrasive grains are to be made into a grinding wheel using a suitablebinder, in general it is preferable that the grain sizes beapproximately 140-170 mesh or 50-60 mesh.

Prior to immersion, it is not necessary to perform any preliminarytreatment of the diamond abrasive grains or the hard BN abrasive grains.However, if degreasing or similar treatment is performed, the adhesionof the coating to be formed on the grains can be further increased.

As explained earlier, the formation of a coating by immersion inaccordance with present invention can also be performed on abrasivegrains which have been previously coated with a suitable metal by CVD,vacuum sputtering, or other method.

However, the previously-applied coating is not restricted to a coatingof a metal, and it can be a coating of a metallic compound in accordancewith the present invention. Namely, by treating the surface of theabrasive grains which were coated with a metallic compound, e.g., ametallic carbide in the above manner by second time using the same typeof molten salt bath, the metal in the coating which is already formed onthe surface of the abrasive grains combines with another metal in thebath, and a second coating is formed. In this case, the metal in thebath which combines with the metal in the coating is preferably B, whichis present in the bath in the form of an oxide such as B₂ O₃. As aresult, the second coating is a metallic boride coating. Namely, anoxide is boron (such as boron oxide) reacts in the fluoride-containingmolten salt bath (e.g., B₂ O₃ →KBF₄). The resulting compound reacts witha metal (such as Ti) in the first coating to form a substance MB₂ or MB(such as TiB₂ or TiB), wherein M stands for a metal. As a result of thistreatment, the oxidation resistance of the abrasive grains is furtherimproved.

Next, the present invention will be described more concretely on thebasis of the following examples. In the examples, unless otherwiseindicated, % refers to % by weight.

Example 1

42.2 mole % of KCl, 20.2 mole % of BaCl₂, and 37.6 mole % of NaF wereplaced in a porcelain crucible and then melted by heating. Next,prescribed quantities of metallic oxides and alloy powders (ferroalloys)were added and the mixtures was thoroughly stirred to obtain animmersion bath.

2 g of diamond abrasive grains having a grain size of 50-60 mesh(tradename "MBS" manufactured by General Electric) were immersed in thisimmersion bath at 850° C. for 2 hours. The average coating thickness was1.8 μm. The types and quantities of metallic oxides and alloys used inthis example as well as the composition of the resulting coating areshown in Table 1.

The immersion of the abrasive grains was performed using a stainlesssteel wire mesh bucket.

The resulting coated abrasive grains were placed together with 14sintered carbide balls having a diameter of 4.6 mm into a tempered hardglass bottle (16.5 mm in diameter, 30 ml). The glass bottle was rotatedat 150 rpm for 15 minutes, after which the coating adhesion wasevaluated by comparing the weight of coating which had peeled off to theinitial weight. The results are shown in Table 1.

FIG. 1 and FIG. 2 are graphs which show the DTA (differential thermalanalysis) curve and the TGA (thermogravimetric analysis) curve foruncoated diamond abrasive grains (FIG. 1) and for abrasive grains whichwere coated in the above manner with a caride of chromium (Test No. 2,FIG. 2).

If the curves of FIG. 2 for coated abrasive grains of the presentinvention are compared with the curves of FIG. 1 for a conventionalexample, it can be seen that the temperature which shows the peak ofheat flow increased from 904° C. to 1018° C., and the temperature atwhich oxidation loss occurred also increased. It is therefore clear thatthe oxidation resistance of the abrasive grains of the present inventionis far superior to that of the conventional example.

FIGS. 3a and 3b are photomicrographs (×45) showing the state of thesurface coating and of peeled material from diamond abrasive grainscoated with a TiC coating (formed in a manner similar to that used forTest No. 1) after the completion of the above-described peeling test.There was substantially no peeling of the coating, and the onlyreduction in the size of the grains was due to chipping. The coatedabrasive grains of FIGS. 3a, 3b were prepared by immersion at 900° C.for 3 hours, and the thickness of the TiC coating was 5 μm.

Similarly, FIGS. 4a and 4b are photomicrographs (×45) showing the stateof the surface coating and of peeled material after the completion of apeeling test of diamond abrasive grains which were not coated with acoating in accordance with the present invention. It can be seen thatthe abrasive grains of FIGS. 4a, 4b underwent a greater reduction insize than did the abrasive grains of FIGS. 3a, 3b, demonstrating thatthe coating of the present invention is effective not only forpreventing peeling but also for preventing chipping.

FIG. 5 is an electron microscope photograph (×1500) of the surface of apiece of the coated abrasive grains of FIG. 3a. It clearly shows thatthe TiC coating is adhered to the surface of the abrasive grains.

FIG. 6 is a photograph (×3000) which was produced by an X-raymicroanalyzer and which shows the distribution of elemental Ti in thesame coated abrasive grain. It can be seen that Ti is adequatelydispersed among the diamond abrasive grains.

Coated diamond abrasive grains in accordance with the present inventionwhich were obtained in the above-described manner were used as astarting material powder for the manufacture of a metal-bonded diamondwhetstone. It was found to have a working life of roughly three timesthat of a metal-bonded diamond whetstone made from conventional uncoateddiamond abrasive grains.

Example 2

The coated diamond abrasive grains of Example 1 were coated with asecond layer in the form of boride coating using the method of thepresent invention. Namely, B₂ O₃ and boron in elemental form were addedto the above-described molten salt bath to prepare an immersion bath,and then the procedure of Example 1 was followed. The same peelingresistance test as for Example 1 was performed on the resulting abrasivegrains. The compositions of the resulting coatings and the test resultsare shown in Table 2.

Immersion was carried out at 850° C. for one hour. Each of the coatingthickness shown in Table 2 is the total of the first layer (the carbidecoating) and the second layer (the boride coating).

Example 3

In this example, diamond abrasive grains were employed which had beenpreviously coated with a metallic layer (2 μm thick) of Ti, Cr, V, W, orMo using chemical vapor deposition. The previously-coated grains werethen coated with a boride coating in the same manner as in Example 2.The compositions of the coatings and the peeling characteristics of theresulting coated abrasive grains are shown in Table 3. Immersion wasperformed at 950° C. for 2 hours.

Example 4

In this example, a metallic layer (approximately 2 μm thick) of Ti, Cr,V, or Mo was formed by chemical vapor deposition on hard BN abrasivegrains (trade name "BZN-550" manufactured by General Electric) having agrain size of 50-60 mesh. Next, a boride coating was formed atop thegrains in the same manner as in Example 2. The compositions of thecoatings and the peeling characteristics of the resulting coatedabrasive grains are shown in Table 4. Immersion was performed at 950° C.for 2 hours.

Example 5

In this example, SiC abrasive grains (420-350 μm) were coated with acarbide coating in the manner of Example 1. The immersion bathtemperature was maintained at 950° C., and the SiC abrasive grains werereacted therein for 2 hours. As in Example 1, the immersion bathcontained a metallic oxide and an alloy. However, in this example thesubstitution reaction SiC→MeC (Me:metal) took place, and a carbidecoating was formed.

The types of metallic oxides and alloys employed in the immersion bathare shown in Table 5. The table also shows the coating thickness and theamount of peeling (%) of the coating.

Example 6

In this example, a nitride coating was applied to C-BN abrasive grains(210-177 μm) in the manner of Example 1. Reaction was performed at 950°C. for 2 hours.

As when forming a carbide coating, the immersion bath contained ametallic oxide and an alloy. The substitution reaction BN→MeN (Me:metal)took place, and a nitride coating was formed. The composition of thecoating and its characteristics are shown in Table 6.

Example 7

In this example, first a nitride coating was formed on BN abrasivegrains (210-177 μm) in the manner of Example 6, and then a boride layerwas formed atop the nitride coating in the manner of Example 2.Immersion was performed at 950° C. for one hour.

The results for this example are shown in Table 7.

Example 8

In this example, a bromide bath was employed. The bath composition was37.6 mole % of NaF, 31.2 mole % of NaBr, and 31.2 mole % of KBr. MBSabrasive grains (50-60 mesh) were immersed in the bath at 950° C. for 2hours.

The results are shown in Table 8.

Example 9

In this example, an immersion bath was employed which had a compositionof 37.6 mole % of NaF, 31.2 mole % of NaI, and 31.2 mole % of KI. Ametallic oxide, a metallic halide, a metal in elemental form, or analloy was added to the bath, and various coatings were formed on MBSabrasive grains (50-60 mesh) in a manner like that employed inExample 1. Immersion was carried out at 950° C. for 2 hours.

The results are shown in Table 9.

Example 10

In this example, an immersion bath was employed which had a compositionof 37.6 mole % of NaF and 62.4 mole % of KF. A metallic oxide, a metalin elemental form, or an alloy was added to the bath, and variouscoatings were formed on MBS abrasive grains (50-60 mesh) in a mannerlike that employed in Example 1. Immersion was carried out at 950° C.for 2 hours.

The results are shown in Table 10.

In this manner, according to the present invention, by merely performingsimple immersion treatment using a molten salt bath which is easy tohandle, a surface coating which has excellent adhesion and oxidationresistance not exhibited by conventional coatings is obtained.Therefore, the present invention greatly contributes to the advancementof industry.

                  TABLE 1                                                         ______________________________________                                        (EXAMPLE 1)                                                                                          Coating Coating Amount                                 Test Metallic          Compo-  Thickness                                                                             of Peeling                             No.  Oxide    Alloy    sition  (μm) (%)                                    ______________________________________                                        1    TiO.sub.2                                                                              Fe--Ti   Ti-C    2.0     0.5                                    2    Cr.sub.2 O.sub.3                                                                       Fe--Cr   Cr--C   1.8     0.4                                    3    V.sub.2 O.sub.5                                                                        Fe--V    V--C    1.8     0.3                                    4    WO.sub.3 Fe--W    W--C    1.9     0.4                                    5    MoO.sub.3                                                                              Fe--Mo   Mo--C   1.8     0.5                                    6    Ta.sub.2 O.sub.5                                                                       Fe--Ta   Ta--C   1.5     0.3                                    7    ZrO.sub.2                                                                              Fe--Zr   Zr--C   1.4     0.2                                    8    Nb.sub.2 O.sub.5                                                                       Fe--Nb   Nb--C   1.8     0.3                                    9    SiO.sub.2                                                                              Si       Si-C    0.8     0.3                                    10   B.sub.2 O.sub.3                                                                        B        B--C    0.5     0.4                                    ______________________________________                                         NOTE: The thickness of the coating was determined on the surface of a         piece of the peeled grains by means of an electron microscope.           

                  TABLE 2                                                         ______________________________________                                        (EXAMPLE 2)                                                                        First                   Total                                                 Layer                   Thickness                                                                             Amount                                   Test Compo-   Second Layer   of Coatings                                                                           of Peeling                               No.  sition   Composition    (μm) (%)                                      ______________________________________                                        11   Ti--C    Ti--B single layer or                                                                        2.1     0.6                                                    Ti--B, B--C                                                                   composite                                                       12   Cr--C    Cr--B single layer or                                                                        1.8     0.5                                                    Cr--B, B--C                                                                   composite                                                       13   V--C     V--B single layer or                                                                         1.9     0.4                                                    V--B, B--C                                                                    composite                                                       14   W--C     W--B single layer or                                                                         1.7     0.5                                                    W--B, B--C                                                                    composite                                                       15   Mo--C    Mo--B single layer or                                                                        1.7     0.5                                                    Mo--B, B--C                                                                   composite                                                       16   Ta--C    Ta--B single layer or                                                                        1.4     0.4                                                    Ta--B, B--C                                                                   composite                                                       17   Zr--C    Zr--B single layer or                                                                        1.3     0.4                                                    Zr--B, B--C                                                                   composite                                                       18   Nb--C    Nb--B single layer or                                                                        1.7     0.5                                                    Nb--B, B--C                                                                   composite                                                       ______________________________________                                         NOTE: The second layer was very thin (0.1˜0.2 μm). A total           thickness smaller than the starting layer seems to be caused by               experimental errors.                                                     

                  TABLE 3                                                         ______________________________________                                        (EXAMPLE 3)                                                                                              Total Thickness                                                                         Amount                                   Test First Layer                                                                              Second Layer                                                                             of Coatings                                                                             of Peeling                               No.  Composition                                                                              Composition                                                                              (μm)   (%)                                      ______________________________________                                        19   Ti         Ti--B      1.9       3.0                                      20   Cr         Cr--B      1.7       2.8                                      21   V          V--B       1.6       2.6                                      22   W          W--B       1.8       3.2                                      23   Mo         Mo--B      1.6       3.5                                      ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        (EXAMPLE 4)                                                                                              Total Thickness                                                                         Amount                                   Test First Layer                                                                              Second Layer                                                                             of Coatings                                                                             of Peeling                               No.  Composition                                                                              Composition                                                                              (μm)   (%)                                      ______________________________________                                        24   Ti         Ti--B      1.8       3.1                                      25   Cr         Cr--B      1.7       2.6                                      26   V          V--B       1.5       2.7                                      27   W          W--B       1.8       3.3                                      28   Mo         Mo--B      1.6       3.7                                      ______________________________________                                    

                                      TABLE 5                                     __________________________________________________________________________    (EXAMPLE 5)                                                                                             Coating                                                                             Amount                                        Test                                                                             Metallic               Thickness                                                                           of Peeling                                    No.                                                                              Oxide                                                                              Alloy                                                                              Coating Composition                                                                        (μm)                                                                             (%)                                           __________________________________________________________________________    29 TiO.sub.2                                                                          Fe--Ti                                                                             Ti--C single layer or                                                                      23.0  1.0                                                        Ti--C, Ti--Si composite                                          30 Cr.sub.2 O.sub.3                                                                   Fe--Cr                                                                             Cr--C single layer or                                                                      25.0  0.9                                                        Cr--C, Cr--Si composite                                          31 V.sub.2 O.sub.5                                                                    Fe--V                                                                              V--C single layer or                                                                       5.5   0.6                                                        V--C, V--Si composite                                            32 WO.sub.3                                                                           Fe--W                                                                              W--C single layer or                                                                       5.6   0.7                                                        W--C, W--Si composite                                            33 MoO.sub.3                                                                          Fe--Mo                                                                             Mo--C single layer or                                                                      5.3   1.0                                                        Mo--C, Mo--Si composite                                          34 Ta.sub.2 O.sub.5                                                                   Fe--Ta                                                                             Ta--C single layer or                                                                      4.5   0.6                                                        Ta--C, Ta--Si composite                                          35 ZrO.sub.2                                                                          Fe--Zr                                                                             Zr--C single layer or                                                                      4.4   0.5                                                        Zr--C, Zr--Si composite                                          36 Nb.sub.2 O.sub.5                                                                   Fe--Nb                                                                             Nb--C single layer or                                                                      5.6   0.5                                                        Nb--C, Nb--Si composite                                          __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    (EXAMPLE 6)                                                                                             Coating                                                                             Amount                                        Test                                                                             Metallic               Thickness                                                                           of Peeling                                    No.                                                                              Oxide                                                                              Alloy                                                                              Coating Composition                                                                        (μm)                                                                             (%)                                           __________________________________________________________________________    37 TiO.sub.2                                                                          Fe--Ti                                                                             Ti--N single layer or                                                                      9.2   1.2                                                        Ti--N, Ti--B composite                                           38 Cr.sub.2 O.sub.3                                                                   Fe--Cr                                                                             Cr--N single layer or                                                                      10.0  1.0                                                        Cr--N, Cr--B composite                                           39 V.sub.2 O.sub.5                                                                    Fe--V                                                                              V--N single layer or                                                                       4.5   0.8                                                        V--N, V--B composite                                             40 WO.sub.3                                                                           Fe--W                                                                              W--N single layer or                                                                       4.0   0.5                                                        W--N, W--B composite                                             41 MoO.sub.3                                                                          Fe--Mo                                                                             Mo--N single layer or                                                                      4.0   0.5                                                        Mo--N, Mo--B composite                                           42 Ta.sub.2 O.sub.5                                                                   Fe--Ta                                                                             Ta--N single layer or                                                                      4.5   0.4                                                        Ta--N, Ta--B composite                                           43 ZrO.sub.2                                                                          Fe--Zr                                                                             Zr--N single layer or                                                                      4.3   0.3                                                        Zr--N, Zr--B composite                                           44 Nb.sub.2 O.sub.5                                                                   Fe--Nb                                                                             Nb--N single layer or                                                                      4.5   1.0                                                        Nb--N, Nb--B composite                                           __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    (EXAMPLE 7)                                                                                                Total Thickness                                                                       Amount                                   Test                                                                             First Layer  Second Layer of Coatings                                                                           of Peeling                               No.                                                                              Composition  Composition  (μm) (%)                                      __________________________________________________________________________    45 Ti--N single layer or                                                                      Ti--B single layer or                                                                      9.2     1.3                                         Ti--N, Ti--B composite                                                                     Ti--B, B--N composite                                         46 Cr--N single layer or                                                                      Cr--B single layer or                                                                      9.9     1.2                                         Cr--N, Cr--B composite                                                                     Cr--B, B--N composite                                         47 V--N single layer or                                                                       V--B single layer or                                                                       4.6     1.0                                         V--N, V--B composite                                                                       V--B, B--N composite                                          48 W--N single layer or                                                                       W--B single layer or                                                                       3.9     0.7                                         W--N, W--B composite                                                                       W--B, B--N composite                                          49 Mo--N single layer or                                                                      Mo--B single layer or                                                                      4.0     0.6                                         Mo--N, Mo--B composite                                                                     Mo--B, B--N composite                                         50 Ta--N single layer or                                                                      Ta--B single layer or                                                                      4.4     0.5                                         Ta--N, Ta--B composite                                                                     Ta--B, B--N composite                                         51 Zr--N single layer or                                                                      Zr--B single layer or                                                                      4.3     0.5                                         Zr--N, Zr--B composite                                                                     Zr--B, B--N composite                                         52 Nb--N single layer or                                                                      Nb--B single layer or                                                                      4.4     1.2                                         Nb--N, Nb--B composite                                                                     Nb--B, B--N composite                                         __________________________________________________________________________     NOTE: The second layer was very thin (0.1˜0.2 μm). A total           thickness smaller than the starting layer seems to be caused by               experimental errors.                                                     

                  TABLE 8                                                         ______________________________________                                        (EXAMPLE 8)                                                                                                Coating Amount                                   Test Additive to  Coating    Thickness                                                                             of Peeling                               No.  Immersion Bath                                                                             Composition                                                                              (μm) (%)                                      ______________________________________                                        53   Ti           Ti--C      6.2     1.1                                      54   Fe-- Ti      Ti--C      6.1     1.0                                      55   V.sub.2 O.sub.5, Al                                                                        V--C       3.0     0.7                                      56   K.sub.2 TiF.sub.6, Fe--Ti                                                                  TiC        6.3     1.1                                      57   Fe--Cr       Cr--C      5.3     0.9                                      58   Fe--V        V--C       5.4     0.4                                      59   Fe--W        W--C       5.3     0.8                                      60   Fe--Mo       Mo--C      5.2     0.9                                      61   ZrO.sub.2, Fe--Zr                                                                          Zr--C      4.0     0.4                                      62   Nb.sub.2 O.sub.5, Fe--Nb                                                                   Nb--C      5.3     0.4                                      ______________________________________                                         NOTE: It is thought that the surface of commonlyused metals and alloys is     slightly oxidized, and technically, these substances are not pure metals.

                  TABLE 9                                                         ______________________________________                                        (EXAMPLE 9)                                                                                                Coating Amount                                   Test Additive to  Coating    Thickness                                                                             of Peeling                               No.  Immersion Bath                                                                             Composition                                                                              (μm) (%)                                      ______________________________________                                        63   Ti           Ti--C      6.1     1.0                                      64   V.sub.2 O.sub.5, Al                                                                        V--C       3.0     0.7                                      65   K.sub.2 TiF.sub.6, Fe--Ti                                                                  Ti--C      5.4     1.0                                      66   Fe--Cr       Cr--C      5.2     0.9                                      67   Fe--V        V--C       5.3     0.5                                      68   ZrO.sub.2, Fe--Zr                                                                          Zr--C      4.0     0.5                                      69   Nb.sub.2 O.sub.5, Fe--Nb                                                                   Nb--C      5.1     0.4                                      ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                        (EXAMPLE 10)                                                                                               Coating Amount                                   Test Additive to  Coating    Thickness                                                                             of Peeling                               No.  Immersion Bath                                                                             Composition                                                                              (μm) (%)                                      ______________________________________                                        70   Ti           Ti--C      10.1    1.5                                      71   V.sub.2 O.sub.5, Al                                                                        V--C       4.5     0.9                                      72   Fe--Cr       Cr--C      8.9     1.2                                      73   Fe--V        V--C       8.1     1.3                                      74   ZrO.sub.2, Fe--Zr                                                                          Zr--C      6.3     0.6                                      75   Nb.sub.2 O.sub.5, Fe--Nb                                                                   Nb--C      7.8     0.7                                      76   TiCl.sub.3   Ti--C      2.7     0.8                                      ______________________________________                                    

We claim:
 1. Coated abrasive grains comprising abrasive grains whosesurfaces are coated with a coating comprising at least one substanceselected from carbides, borides, and nitrides of a metal, said coatingbeing applied by an immersion method using a fluoride-containing moltensalt bath.
 2. Coated abrasive grains as defined in claim 1, wherein saidabrasive grains which are coated are diamond abrasive grains or hard BNabrasive grains.
 3. Coated abrasive grains as defined in claim 1,wherein the carbides, borides, and nitrides are of at least one metalselected from the group consisting of the numbers of Group IVa, Va, andVIa of the periodic table.
 4. Coated abrasive grains as defined in claim1, wherein the carbides, borides, and nitrides are of at least one metalselected from the group consisting of Si, Cv, V, B, W, Mo, Ti, Zr, Hf,Nb and Ta.
 5. Coated abrasive grains as defined in claim 3, wherein saidabrasive grains which are coated are diamond abrasive grains or hard BNabrasive grains.
 6. Coated abrasive grains as defined in claim 4,wherein said abrasive grains which are coated are diamond abrasivegrains or hard BN abrasive grains.
 7. Coated abrasive grains as definedin claim 1, wherein said immersion method comprises preparing animmersion bath comprising a molten salt bath containing said metal, andimmersing abrasive grains to be treated in said immersion bath for asuitable length of time.
 8. Coated abrasive grains as defined in claim2, wherein said immersion method comprises preparing an immersion bathcomprising a molten salt bath containing said metal, and immersingabrasive grains to be treated in said immersion bath for a suitablelength of time.
 9. Coated abrasive grains as defined in claim 3, whereinsaid immersion method comprises preparing an immersion bath comprising amolten salt bath containing said metal, and immersing abrasive grains tobe treated in said immersion bath for a suitable length of time. 10.Coated abrasive grains as defined in claim 4, wherein said immersionmethod comprises preparing an immersion bath comprising a molten saltbath containing said metal, and immersing abrasive grains to be treatedin said immersion bath for a suitable length of time.
 11. Coatedabrasive grains as defined in claim 1, wherein the fluoride-containingmolten salt bath is a KCl--BaCl₂ --NaF bath, and the proportion offluoride is not more than 50 mol %.
 12. Coated abrasive grainscomprising abrasive grains whose surfaces are coated with a coatingcomprising at least one substance selected from carbides, borides, andnitrides of a metal, said coating having been formed by a chemicalreaction between the surface of the abrasive grains and a molten saltbath.